Process for enhanced propylene yield from cracked hydrocarbon feedstocks and reduced benzene in resulting naphtha fractions

ABSTRACT

A process in which a catalytic cracking unit is operated to crack a hydrocarbon feedstock in a manner to enhance light olefin yields. The accompanying benzene-containing naphtha product stream is further processed through a benzene selective membrane to provide a low content benzene stream. Refiners frequently operate their cracking units to optimize light olefin yields, e.g. propylene, in response to needs in the petrochemical industry, and it has been discovered that units operated in this manner frequently produce naphtha containing increased amounts of benzene. The method of this invention therefore allows one to operate the unit when it is desired to optimize light olefin yields, yet at the same time produce a naphtha yield having a low benzene content. The invention is particularly useful when the cracking unit utilizes pentasil zeolites at increased concentrations to enhance light olefins yield.

PRIORITY CLAIM OF EARLIER NATIONAL APPLICATIONS

This application is a national stage entry under 35 U.S.C. §371 ofInternational Application No. PCT/US2009/005095 filed Sep. 11, 2009which claims benefit of U.S. Provisional Application No. 61/192,074filed Sep. 15, 2008.

BACKGROUND OF THE INVENTION

The invention relates to catalytic cracking processes conducted toproduce light olefins. The invention further relates to methods forreducing benzene in naphtha fractions produced in such processes.

Benzene is a known carcinogen that arises in the production of gasoline.Regulations in the European Union, US, and other locations require lessthan 1% benzene in gasoline. Gasoline is produced at a refinery byblending component streams, including butane, isopentane, alkylate,isomerate, straight run naphtha, hydrocrackate, catalytic naphtha, steamcracked naphtha, coker naphtha, pyrolysis gasoline, catalytic reformate,vacuum gas oil, and oxygenates. The naphthas formed from catalyticcracking, e.g., fluidized catalytic cracking (FCC), can be furtherfractionated into light cat naphtha, intermediate cat naphtha, and heavycat naphtha.

The product distribution from current FCC processes comprises a numberof constituents in addition to gasoline naphtha. While gasoline is ofprimary interest to most refiners, light olefins and LPG are also foundin the FCC product, and are increasingly becoming of interest torefiners as those products become more valuable. The light olefinsproduced can be used for a number of purposes, e.g., they are upgradedvia sulfuric or HF alkylation to high quality alkylate. LPG is used forcooking and/or heating purposes. Accordingly, operators of FCC units canvary the content of their products depending upon the markets they areserving and the value associated with each of the components found in anFCC product.

Propylene is a particular light olefin in high demand. It is used inmany of the world's largest and fastest growing synthetic materials andthermoplastics. Refiners are relying more and more on their FCC units tomeet the increased demand for propylene, thus shifting the focus of thetraditional FCC unit away from transportation fuels and more towardpetrochemical feedstock production as operators seek opportunities tomaximize margins. Indeed, the FCC unit provides one third of the world'spropylene, and being able to increase propylene output from the unit isof value when propylene prices are high.

If a refinery cannot expand its existing FCC unit, the unit's operatorshave rather limited options for increasing light olefin production.Reported options include: (a) using additive ZSM-5 catalyst and/oradditive in the FCC unit; and (b) increasing the severity of theconditions, e.g., temperature, in the unit, e.g., production of crackedgas from gas oil over pentasil zeolites, e.g., ZSM-5.

It has been noted, however, that processes such as the above typicallyproduce a product that, when fractionated to the gasoline naphthastreams, have higher concentrations of benzenes compared to units run atconditions to maximize gasoline yields. Naphtha fraction from a FCC unitoperated to enhance light olefin yields can contain more than 2%benzene. The source of increased benzene is not readily recognized byrefiners.

Accordingly, there can be reluctance to rely on FCC units forsubstantially meeting olefin needs, or reluctance to maximize the use ofthe pentasil additive catalysts. The reluctance is further reinforcedgiven that when refiners use pentasil additives to enhance olefinyields, gasoline yields are often sacrificed. In other words, therefiner is facing the additional issue that the yield of a valuableproduct is being reduced in addition to the fact the process willrequire processing the product to remove the increased amount ofbenzene.

Polymeric membranes have been reported to separate aromatics.

U.S. Pat. No. 2,930,754 (Stuckey), U.S. Pat. No. 2,958,656 (Stuckey),U.S. Pat. No. 3,370,102 (Carpenter et al.), U.S. Pat. No. 4,115,465(Elfert et al.), U.S. Pat. No. 4,944,880 (Ho et al.), U.S. Pat. No.5,028,685 (Ho et al.), U.S. Pat. No. 5,063,186 (Schucker), and U.S. Pat.No. 5,635,055 (Sweet et al.) all relate to membranes foraromatic/non-aromatic separations, but none address benzene removal fromhydrocarbon streams.

U.S. Pat. No. 6,180,008 (White) and U.S. Pat. No. 6,187,987 (Chin etal.) refers to polyimide membranes and processes using hyperfiltrationto recover aromatic solvents. Benzene removal from hydrocarbon streams,however, is not addressed.

U.S. Pat. No. 5,914,435 (Streicher and Prevost) describe a process wherea sidestream from a distillation column enriched in benzene is treatedwith a membrane permeation zone in order to reduce the benzene contentof the treated hydrocarbon stream. The membrane is selective forbenzene, and at least part of the retentate low in benzene is dividedinto two streams and recycled to two different levels in thedistillation column. It is believed that the distillation column is anaphtha pre-fractionating column designed to separate C₅ to C₁₀hydrocarbons, wherein the those hydrocarbons having a boiling point inthe range of 150° to 200° C. are collected at the bottom of the columnand hydrocarbons having a boiling point of about 50° C. are collectedoff the top of the column.

A publication titled “Reduce Your Tier 2 Gasoline Compliance Costs withGrace Davison S-Brane™ Technology” and presented at the Spring 2002 NPRAmeeting (AM-02-21) by J. Balko describes reducing sulfur content ingasoline by employing S-Brane® membranes. See also U.S. Pat. No.6,896,796 (White, Wormsbecher, and Lesemann). Balko generally mentionsretentate aromatics level (particularly benzene) is substantiallyreduced by the process using the S-Brane membrane, but there is nomention of doing so in connection with a gasoline stream relating toolefin production, and sulfur reduction was the primary purpose of usingthe S-Brane membrane. Indeed, Balko does not provide supporting data onbenzene removal.

The following references also describe using membranes to remove sulfurfrom hydrocarbon feeds. Except for the '761 to Balko, these referencesdo not mention benzene removal. U.S. Pat. No. 6,649,061 (Minhas et al.);U.S. Pat. No. 7,048,846 (White et al.); and U.S. Pat. No. 7,267,761(Balko).

Jonquieres, R. Clement, P. Lochon, J. Neel, M. Dresch, and B. Chretien;“Industrial state-of-the-art of pervaporation and vapour permeation inthe western countries”; J. MEMBRANE SCI. 206 (2002) 87-117, states that“the petrochemical industry is now considering these new separationprocesses as very good candidates to take up the coming world-widechallenge of aromatics removal from gasoline that remains one of thecurrent great issues of public health”, but no additional references orinformation is given. See also, A. Jonquieres, R. Clement, and P.Lochon; “Permeability of block copolymers to vapors and liquids”; PROG.POLYM. SCI. 27 (2002) 1803-1877.

U.S. Pat. No. 6,232,518 (Ou) refers to using cyclodextrins for removalof benzene from hydrocarbon streams.

J. Garci Villaluenga and A. Tabe-Mohammadi; “A review of the separationof benzene/cyclohexane mixtures by pervaporation processes”; J. MEMBRANESCI. 169 (2000) 159-174 reviews existing technologies for recovery ofbenzene. Removal of benzene to low levels, however, is not addressed.

New regulations are calling for lower levels of benzene in gasoline.Since the FCC unit produces blending components for gasoline, keepingthe benzene levels low is critical to a refiner. It would therefore bedesirable to have a process that allows for increased propyleneproduction while simultaneously lowering the benzene levels in FCCgasoline produced during that production. As evidenced from abovediscussion, a practical solution to this dilemma has not been disclosedor suggested.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic illustration showing one embodiment of the inventionwherein light cat naphtha is treated with a membrane to reduce theamount of benzene in the stream.

FIG. 2 is a graph illustrating the invention's performance in reducingbenzene concentrations with respect to membranes M1 and M2 described inExample 3.

SUMMARY OF INVENTION

We have found that a membrane process can be used to address theundesirable buildup of benzene when cracking feedstocks to lightolefins, e.g., propylene, using pentasil zeolite catalysts or otherconditions known to enhance light olefin yields, thereby allowing forprocess conditions that improves light olefin yield from an FCC unit,while at the same time providing a more environmentally friendly naphthastream that can be blended into the gasoline pool. Light olefins areintended herein to mean ethylene, propylene and butylenes, i.e., olefinscontaining two to four carbons (C₂ to C₄).

The invention comprises introducing a hydrocarbon feedstock into areaction zone of a catalytic cracking unit, which feedstock ischaracterized as having an initial boiling point from about 120° C. withend points up to about 850° C. The feedstock is cracked in the reactionzone of the unit by contacting the feedstock with a cracking catalystunder conditions of temperature, catalyst-to-oil ratio, pressure, steamdilution and space velocity such that a light olefin yield from the unitis enhanced compared to that of the same unit operating at more typicalconditions, e.g., those listed below.

Temperature, ° C. 500-535 Cat./Oil  5 to 10 Pressure, atmospheres 1 to 4Steam Dilution, wt % of feed 1 to 5 WHSV, hr⁻¹ 125-200 Pentasil zeolitecrystal 0-1 content, wt % catalyst

The unit thereby produces a product comprising light olefin, naphtha andbenzene, wherein the product comprises about 6 to about 20% propylenebased on the weight of the hydrocarbon feedstock introduced in thecracking step above. The product is then fractionated into at least anlight olefin-containing fraction, and a benzene-containing naphthafraction. Such streams can include full range cat naphtha (boiling pointin the range of 50° C. to about 220° C., and 0.6 to about 3% by weightbenzene) or light cat naphtha (boiling point in the range of 50° C. toabout 105° C., and 1.2 to about 6% by weight benzene).

The light olefin-containing fraction is recovered, and thebenzene-containing naphtha fraction is contacted with a membrane. Themembrane selected for use is benzene selective, and therefore shouldhave a sufficient flux and selectivity to separate from the naphtha abenzene-enriched permeate fraction and a benzene deficient naphtharetentate fraction, said benzene-enriched permeate fraction beingenriched in benzene compared to the retentate. One then recovers thebenzene deficient naphtha retentate fraction, and routes thebenzene-enriched permeate fraction for further processing.

In operating such a process, one has at its disposal a method ofproducing light olefins and low benzene-containing naphtha in acatalytic cracking unit when the unit's conditions are selected toenhance light olefin yield in the catalytic cracking unit, e.g.,conditions such as type and composition of catalyst, temperature,catalyst-to-oil ratio, pressure, steam dilution and/or space velocity.

Being able to remove benzene in this fashion allows a refiner tomaximize its light olefin yield from a FCC product stream. When the unitis utilizing pentasil zeolite for its olefin production, the stream cancontain propylene in the range of 6 to about 20% by weight propylenebased on the weight of the feedstock to the FCC unit.

Accordingly, aromatic selective membranes that preferentially removebenzene from gasoline feedstocks, provide for simultaneous increasedyield of light olefins, e.g., propylene, from an FCC unit while stillproducing a large fraction of gasoline with less than 1% benzene levels,and preferably less than 0.6%. Pervaporation with an aromatic selectivemembrane is a preferred process for removing benzene.

Catalysts and process conditions can moreover be adjusted in an FCC unitto increase the yield of C₃ and C₄ light olefins to greater than 20weight % from a lights stripping tower. Even though such a processleaves a naphtha fraction that can contain more than 1% benzene, themembrane step of the invention splits the gasoline fraction into a majorfraction, the retentate, with less than 1% benzene, and a minorfraction, the permeate, containing greater than 1% benzene. The majorretentate fraction can be directly used in the gasoline pool, while theminor permeate fraction is sent for further processing in the refinery.

DETAILED DESCRIPTION OF THE INVENTION Catalytic Cracking Processes

The catalytic cracking process of this invention is preferably a FCCprocess. Catalysts used in FCC processes are in particle form, usuallyhave an average particle size in the range of 20 to 200 microns, andcirculate between a cracking reactor and a catalyst regenerator. In thereactor, hydrocarbon feed contacts hot, regenerated catalyst thatvaporizes and cracks the feed. A FCC unit can be operated under a rangeof conditions, wherein the reaction temperatures range from about 400°to 700° C. with regeneration occurring at temperatures of from about500° to 900° C. The particular conditions depend on the petroleumfeedstock being treated, the product streams desired and otherconditions well known to refiners. For example, lighter feedstock can becracked at lower temperatures. The catalyst (i.e., inventory) iscirculated through the unit in a continuous manner between catalyticcracking reaction and regeneration while maintaining the equilibriumcatalyst in the reactor. The invention can be employed in a FCC unitunder conventional cracking conditions. Typical conditions found in aFCC unit are listed below.

Temperature, ° C. 500-535 Cat./Oil  5 to 10 Pressure, atmospheres 1 to 4Steam Dilution, wt % of feed 1 to 5 WHSV, hr⁻¹ 125-200

Certain embodiments of the invention will utilize conditions that aresomewhat more severe. These more severe processes include those known asDeep Catalytic Cracking (DCC), Catalytic Pyrolysis Process (CPP), andUltra Catalytic Cracking (UCC). Illustrative conditions for the moresevere processes are listed in the table below.

DCC CPP UCC Temperature, ° C. 505-575 560-650 550-570 Cat./Oil  9 to 1515 to 25 18 to 22 Pressure, atmospheres 0.7 to 1.5 0.8 1 to 4 SteamDilution, wt % of feed 10 to 30 30 to 50 20 to 35 WHSV* 0.2 to 20  — 50to 80 *weight hourly space velocity (hr⁻¹)

Those of ordinary skill in the art are familiar as to when suchprocesses can be used with the invention. When the invention is usedwith such processes, certain modifications to the invention may berequired, e.g., activity and attrition may require alteration of thecatalyst, in order to optimize the catalyst composition's effectivenessin those processes. Such modifications are known to those skilled in theart. For example, when using increased amounts of pentasil zeolites suchas ZSM5, the FCC unit can be operated under conventional FCC conditionslisted in the first column of the table above to enhance light olefinyields.

The cracking reaction deposits carbonaceous hydrocarbons or coke on thecatalyst, thereby deactivating it. The cracked products are separatedfrom the coked catalyst. The coked catalyst is stripped of volatiles,usually with steam, in a catalyst stripper and then regenerated. Thecatalyst regenerator burns coke from the catalyst with oxygen containinggas, usually air, to restore catalyst activity and heat catalyst to,e.g., 500° C. to 900° C., usually 600° C. to 750° C. The hot regeneratedcatalyst recycles to the cracking reactor to crack more fresh feed. Fluegas from the regenerator may be treated to remove particulates orconvert CO, and then discharged into the atmosphere. The FCC process,and its development, is described in the Fluid Catalytic CrackingReport, Amos A. Avidan, Michael Edwards and Hartley Owen, in the Jan. 8,1990 edition of the OIL & GAS JOURNAL.

A variety of hydrocarbon feedstocks can be cracked in the FCC unit toproduce light olefins and gasoline. Typical feedstocks include in wholeor in part, a gas oil (e.g., light, medium, or heavy gas oil) having aninitial boiling point above about 120° C. (250° F.), a 50% point of atleast about 315° C. (600° F.), and an end point up to about 850° C.(1562° F.). The feedstock may also include deep cut gas oil, vacuum gasoil, coker gas oil, thermal oil, residual oil, cycle stock, whole topcrude, tar sand oil, shale oil, synthetic fuel, heavy hydrocarbonfractions derived from the destructive hydrogenation of coal, tar,pitches, asphalts, hydrotreated feedstocks derived from any of theforegoing, and the like. As will be recognized, the distillation ofhigher boiling petroleum fractions above about 400° C. must be carriedout under vacuum in order to avoid thermal cracking. The boilingtemperatures utilized herein are expressed in terms of convenience ofthe boiling point corrected to atmospheric pressure. Even high metalcontent resids or deeper cut gas oils having an end point of up to about850° C. can be cracked.

Y-type zeolites are typically used in FCC processes to produce gasoline.These zeolites include zeolite Y (U.S. Pat. No. 3,130,007); ultrastableY zeolite (USY) (U.S. Pat. No. 3,449,070); rare earth exchanged Y (REY)(U.S. Pat. No. 4,415,438); rare earth exchanged USY (REUSY);dealuminated Y (DeAlY) (U.S. Pat. No. 3,442,792; U.S. Pat. No.4,331,694); and ultrahydrophobic Y (UHPY) (U.S. Pat. No. 4,401,556).These zeolites are large-pore molecular sieves having pore sizes greaterthan about 7 Angstroms. In current commercial practice most crackingcatalysts contain these zeolites.

Metal cation exchanged zeolites, e.g., MgUSY, ZnUSY and MnUSY zeolites,can also be employed and are formed by using exchange solutionscontaining the metal salts of Mg, Zn or Mn or mixtures thereof in thesame manner with respect to the formation of REUSY except that a salt ofmagnesium, zinc or manganese is used in lieu of the rare-earth metalsalt used to form REUSY. The content and manufacture of these catalystsare well known in the art.

The amount of Y-type zeolite in the catalyst composition for use in theinvention should be sufficient to produce molecules in the gasolinenaphtha range. In general, zeolite Y will be present in amounts rangingfrom 1 to 99% by weight of the catalyst. Catalysts comprising about 12to about 60% by weight Y-type zeolite are more typical, with specificamounts depending on amount of activity desired. The amount of Y-typezeolite typically is such that the total amount of Y-type zeolite andthe pentasil described below comprises at least about 35% by weight ofthe total catalyst composition.

As indicated above, adding pentasil zeolites to the catalyst inventoryis one method of operating a FCC unit to enhance light olefin yield inaccordance with this invention. These catalysts are well known and arecommonly called additive catalysts. The pentasil zeolites suitable forthis invention include those zeolite structures having a five-memberedring. In preferred embodiments, the catalyst composition of thisinvention comprises one or more pentasils having an X-ray diffractionpattern of ZSM-5 or ZSM-I1. Suitable pentasils include those describedin U.S. Pat. No. 5,380,690, the contents of which are incorporated byreference. Commercially available synthetic shape selective zeolites arealso suitable.

The preferred pentasil zeolites generally have a Constraint Index of1-12. Details of the Constraint Index test are provided in J. CATALYSIS,67, 218-222 (1981) and in U.S. Pat. No. 4,711,710, both of which areincorporated herein by reference. Such pentasils are exemplified byintermediate pore zeolites, e.g., those zeolites having pore sizes offrom about 4 to about 7 Angstroms. ZSM-5 (U.S. Pat. No. 3,702,886 andU.S. Pat. No. Re. 29,948) and ZSM-I1 (U.S. Pat. No. 3,709,979) arepreferred. Methods for preparing these synthetic pentasils are wellknown in the art. The preferred embodiments of pentasil have relativelylow silica-to-alumina ratios, e.g., less than 100:1, preferably lessthan 50:1. A preferred embodiment of this invention has asilica-to-alumina ratio less than 30:1. The pentasil may also beexchanged with metal cations. Suitable metals include those metaldopants described in U.S. Pat. No. 6,969,692 B2, the contents of whichare incorporated by reference. Briefly these metals can be alkalineearth metals, transition metals, rare earth metals, phosphorus, boron,noble metals and combinations thereof. Catalysts comprising ZSM-5pentasils are commercially available from W.R. Grace & Co.-Conn, andsold as Olefins Ultra®, Olefins Extra®, and OlefinsMax® brand catalysts.Olefins Ultra® HZ additive is particularly suitable for use in thisinvention.

Use of these additive catalysts in combination with the macroporouszeolite Y catalysts to enhance light olefins yield is known, and can beused in this invention in accordance with techniques and underconditions known in the art. For example, refiners can add pentasilcontaining catalysts as additive catalysts to their FCC units, with10-80 wt %, typically 12 to 35 wt %, and more typically 25 to 50 wt. %,pentasil zeolite in an amorphous support. In this instance, the pentasilis added as particles that are separate from the particles containingthe conventional large pore zeolite catalysts. These additives aremanufactured to have physical properties that allow them to circulatebetween the reaction zone and regeneration zone with the large porezeolite cracking catalyst. Using pentasil in a separate additive allowsa refiner to retain the ability to use the myriad types of commerciallyavailable large pore zeolite cracking catalyst available today, andallows a refiner to switch between focusing production on gasoline andproduction of light olefins.

Regardless of the pentasil zeolite content in the additive particle, theamount of pentasil zeolite crystal in the total inventory of catalystshould be in a quantity sufficient to enhance olefin yields of the FCCunit compared to when such catalyst are not present, or only lowconcentrations (e.g., 1% by weight or less) are present, e.g., toenhance octane. A “pentasil zeolite crystal” is meant to refer tocrystalline pentasil zeolite in the neat form. The selected pentasilzeolite crystal content, preferably ZSM-5, will preferably be in therange of 2 to about 20% by weight of the catalyst to be used with thisinvention. The amount of the pentasil crystal can be calculated from acatalyst particle containing support and/or matrix utilizing x-raydiffraction techniques known to those skilled in the art.

High pentasil zeolite content catalyst particles will be preferable inthis invention in order to obtain the crystal amount in theaforementioned range. Utilizing the high content particles should avoiddilution of the gasoline cracking activity of the zeolite Y. Such highpentasil zeolite content catalysts are known and described in U.S. Pat.No. 6,916,757, the contents of which are incorporated by reference.

These catalyst additives can be used under a wide range of conditions inthe FCC unit, with light olefins yields depending on the conditionsused. Temperature severity, i.e., higher temperatures, in combinationwith use of pentasil zeolites such as ZSM5 will typically result inenhanced light olefins yields, but frequently also mean lowered gasolineyields with increasing benzene content.

A particularly suitable pentasil zeolite catalyst for use in a FCC unitto enhance olefin yields from a FCC unit is described in WO 2006/050487A1, and US 2008/0093263 A1, the contents of which are incorporated byreference. Briefly, this catalyst is formulated with a Y-type zeolite tocontain pentasil zeolite in a range of about 10% to about 50%, and apentasil zeolite to Y-type zeolite ratio of at least 0.25. The ratio ofpentasil zeolite to Y type zeolite for this catalyst should in generalbe no more than about 3.0. Typical embodiments of the invention compriseabout 10% to about 30% by weight pentasil zeolite, and more typically 10to about 20% by weight. The amount of pentasil zeolite in this catalystis generally such that the amount of pentasil zeolite and Y-type zeolitedescribed above is at least 35% by weight of the total catalystcomposition.

Another suitable pentasil zeolite catalyst for use in this invention isone containing at least 1% by weight iron oxide based on the weight ofthe particles containing the pentasil zeolite. Such catalysts aredescribed in WO 2007/005075 A1, the contents of which are incorporatedby reference. These iron oxide-containing pentasil zeolites typicallycomprise 1 to 10% by weight iron oxide, and the iron in that amount isoutside the framework of the pentasil framework, e.g., iron present inthe pentasil particles' matrix, as opposed to that present in thepentasil's silica alumina framework. WO 2007/005075 A1 describes methodsof preparing these type of pentasil catalysts, and how such catalystscan be incorporated into the catalyst inventory of a FCC unit. Thesecatalysts are particularly designed to enhance the olefin yield of theFCC unit, and this invention would have particular utility with a FCCunit whose catalyst inventory contains such catalysts.

The invention is also suitable for use with an FCC unit whose catalystinventory comprises an iron-based catalyst such as that described in US2006/0011513 A1, wherein the pentasil catalyst comprises a metalphosphate binder, especially those catalysts that comprise an ironphosphate binder.

As mentioned above, light olefins yields in a FCC unit are also enhancedutilizing more severe operations such as DCC, UCC, and CPP listed in thetable above. These methods employ the same catalysts typically used inFCC, but in amounts and ratios tailored to the particular conditionsselected. A range of conditions for each of these operations is providedin the table above. These processes and catalysts used therein are knownin the art. See Chapin et al., “Deep Catalytic Cracking, Maximize OlefinProduction”, presented at 1994 NPRA Annual Meeting, San Antonio, Tex.,Mar. 20-22, 1994 (DCC); Meng et al., “Production of Light Olefins byCatalytic Pyrolysis of Heavy Oil”, PETROLEUM SCIENCE AND TECHNOLOGY Vol.24, pages 413-422, 2006 (CPP); and U.S. Pat. No. 5,846,402 (UCC).

It is also envisioned that the invention can utilize units conducting aprocess known and licensed by Kellogg Brown and Root as the Superflexprocess. In such an embodiment, the Superflex process is by designoperated to enhance light olefin yields when directly processing lightnaphtha feeds, and the product from the unit would then otherwise beprocessed in accordance with the teachings herein.

The hydrocarbon effluent or product from the FCC unit varies and dependsnot only on the feedstock, but also the conditions in the unit. Ahydrocarbon stream processed under typical FCC conditions, as well asthose processed in accordance with the invention will result in producthaving specifications illustrated in the examples below.

The product from the FCC unit is then routed to a fractionation columnfor further processing according to this invention. When the FCC unit isoperated to enhance the light olefin yield in the FCC product, the lightolefins will comprise 6 to about 20% propylene based on the weight ofthe feed to the FCC unit. Table 2 in the aforementioned NPRApresentation regarding DCC processes for maximizing propylene yields isillustrative of the FCC unit product specifications, e.g., about an 8%propylene yield from an FCC unit operated to maximize C₃ olefins. FCCproduct, e.g., about 7% propylene yield, obtained under moreconventional conditions are illustrated in Table 3 of “ReformulatedGasoline: The Role of Current and Future FCC Catalysts”, Young et al.presented at the 1991 NPRA Annual Meeting, Mar. 17-19, 1991, SanAntonio, Tex.

Fractionation

The FCC product is separated into various fractions depending on thefinal product targets for the refinery. For the purposes of thisinvention, the fractionation column, e.g., also known as the FCC MainColumn, separates the FCC product into at least a lightolefin-containing fraction and a naphtha fraction. The lightolefin-containing fraction generally comprises C₄ or lower saturatedand/or unsaturated fractions. The column or fractionator can be thoseknown in the art. See FLUID CATALYTIC CRACKING HANDBOOK-DESIGN,OPERATION, AND TROUBLESHOOTING OF FCC FACILITY, Sadeghbiegi, pp. 18-21(1995) Conditions for running these columns vary depending on the numberof fractions desired. A refiner running a FCC unit will typicallyfractionate the FCC product into a light olefin fraction, gasolinenaphtha fraction, light cycle oil (LCO) fraction, and heavy cycle orbottoms fraction.

Light olefin fractions comprising C₄ or lower saturated and/orunsaturated fractions are typically distilled off as “wet gas”. Wet gasfractions are generally considered those fractions having a boilingpoint of 50° C. or lower. These fractions are typically recovered in acompressor apparatus and processed/distilled into the individual lightolefins flashed from the column.

The gasoline naphtha fraction generally comprises C₅ to C₁₂hydrocarbons. For purposes of this invention, the terms “naphtha” and“gasoline naphtha” are used interchangeably herein to indicatehydrocarbon streams found in refinery operations that have a boilingrange between about 50° C. and about 220° C. The naphtha fractionscontain various amounts of olefinic, aromatic, and non-aromatic, e.g.,aliphatic, hydrocarbon compounds and are primarily differentiated by thefollowing boiling ranges. Light naphthas have a boiling point rangingfrom 50° C. to about 105° C. Intermediate (mid) naphtha has a boilingpoint ranging from 105° C. to about 160° C. Heavy cat naphtha has aboiling point ranging from about 160° C. to about 220° C.

Benzene has a boiling point of 80° C. and significant portions of thebenzene content in the FCC product will distill with the naphthafraction. The naphtha fraction of product from a FCC unit operating toproduce enhanced olefin yields can comprise 0.6 to about 3% by weightbenzene, and frequently above 1% benzene.

If the FCC product is separated into the light, intermediate and heavycat naphtha fractions, significant amounts of benzene will flash withthe light cat naphtha, i.e., the 50 to 105° C. fraction. A light catnaphtha from a FCC unit operating to produce enhanced olefin yields cancomprise 1.2 to about 6% by weight benzene, and frequently above 2%benzene. Such benzene concentrations can be found in light cat naphthaproduced in units in which pentasil zeolite, e.g., ZSM5, crystalcomprises 2 to 20% by weight of the catalyst inventory.

Without being held to a particular theory, it is believed that whenprocesses of higher severity are utilized to produce more light olefinsin a FCC unit, dealkylation of aromatic side chain, cyclization, anddehydrogenation reactions occur to create a greater concentration ofbenzene. Processes relying on pentasil zeolites to produce lightolefins, on the other hand, cracks molecules to olefin molecules thatare removed from the naphtha fraction, thereby resulting in a streamwith higher benzene concentrations. In other words, the pentasilcatalyst removes molecules from the naphtha stream that would otherwisedilute the presence of benzene.

The benzene-containing naphtha fraction, e.g., full range or light, iscollected from the column and then routed to a membrane for furtherprocessing according to the invention. See FIG. 1. The remainingfractions coming off the column, e.g., LCO and HCO, represent,respectively, hydrocarbon fractions in the C₁₂-C₂₂ range and the C₂₂ andhigher range. It is envisioned that these fractions will not typicallyrequire further processing according to this invention. These latterstreams are frequently collected and routed for separate processing, or,e.g., recycle through the FCC unit.

Membrane Separation

Membranes useful in the present invention are those membranes having asufficient flux and selectivity to permeate at least benzene in thepresence of hydrocarbons containing multiple aromatic-compounds, e.g.,benzene-containing gasoline naphtha. Any aromatic selective membrane canbe used. Favorable membranes are chosen on the basis of highproductivity which is the combination of high flux and good aromaticsselectivity, and on the ability to withstand hot operating temperaturestypically in the range of 80° to 120° C. Benzene is highly permeableacross aromatic selective membranes. When you compare the transport rateof benzene across a benzene selective membrane against transport ratesfor compounds of C₅ and above in the feedstock, the relative rate ofbenzene removal is the fastest, thereby making this invention ratherefficient, and at the same time preserving the valuable gasolinefractions in the retentate. Moreover, the membrane systems can bedesigned to handle large-scale feed streams, e.g., on the order of 5405m³ per day (34,000 barrels/day), thereby offering potential economies ofscale to the refiner utilizing this invention.

The membrane will typically have a benzene enrichment factor of greaterthan 1.5, preferably greater than 2, even more preferably from about 2to about 20, most preferably from about 2.5 to 15. Preferably, themembranes have an asymmetric structure, which may be defined as anentity composed of a dense ultra-thin top “skin” layer over a thickerporous substructure of a same or different material. Typically, theasymmetric membrane is supported on a suitable porous backing or supportmaterial.

In one embodiment of the invention, the membrane is a polyimide membraneprepared from a Matrimid® 5218 or a Lenzing polyimide polymer asdescribed in U.S. Pat. No. 6,180,008, the contents of which areincorporated herein by reference.

In another embodiment of the invention, the membrane is one having asiloxane-based polymer as part of the active separation layer, e.g.,coated onto a microporous or ultrafiltration support layer. Examples ofmembrane structure incorporating polysiloxane functionality are found inU.S. Pat. No. 4,781,733; U.S. Pat. No. 4,243,701; U.S. Pat. No.4,230,463; U.S. Pat. No. 4,493,714; U.S. Pat. No. 5,265,734; U.S. Pat.No. 5,286,280; and U.S. Pat. No. 5,733,663; said references being hereinincorporated by reference.

In still another embodiment of the invention, the membrane is anaromatic polyurea/urethane membrane as disclosed in U.S. Pat. No.4,962,271, incorporated herein by reference. Such polyurea/urethanemembranes are characterized as possessing a urea index of at least 20%,but less than 100%, an aromatic carbon content of at least 15 mole %, afunctional group density of at least about 10 per 1000 grams of polymer,and a C═O/NH ratio of less than about 8.

The polyimide, polyurea-urethane, and polysiloxane based membranesdescribed above are particularly useful when separating benzene fromgasoline (e.g., light cat naphtha) produced in a unit whose catalystscomprise pentasil zeolite crystal, e.g., ZSM-5, in a range of 2 to about20% by weight. As mentioned earlier, light cat naphthas produced fromsuch catalysts can comprise 1.2 to 6.0 weight percent benzene.

The membranes can be used in any convenient form such as sheets, tubesor hollow fibers. Sheets can be used to fabricate spiral wound modulesfamiliar to those skilled in the art. Alternatively, sheets can be usedto fabricate a flat stack permeator comprising a multitude of membranelayers alternately separated by feed-retentate spacers and permeatespacers. This device is described in U.S. Pat. No. 5,104,532, hereinincorporated by reference.

Tubes can be used in the form of multi-leaf modules wherein each tube isflattened and placed in parallel with other flattened tubes. Internallyeach tube contains a spacer. Adjacent pairs of flattened tubes areseparated by layers of spacer material. The flattened tubes withpositioned spacer material are fitted into a pressure resistant housingequipped with fluid entrance and exit means. The ends of the tubes areclamped to create separate interior and exterior zones relative to thetubes in the housing. Apparatus of this type is described and claimed inU.S. Pat. No. 4,761,229, herein incorporated by reference.

Hollow fibers can be employed in bundled arrays potted at either end toform tube sheets and fitted into a pressure vessel thereby isolating theinsides of the tubes from the outsides of the tubes. Apparatus of thistype are known in the art. A modification of the standard designinvolves dividing the hollow fiber bundle into separate zones by use ofbaffles, which redirect fluid flow on the tube side of the bundle andprevent fluid channeling and polarization on the tube side. Thismodification is disclosed in U.S. Pat. No. 5,169,530, hereinincorporated by reference.

Multiple separation elements, be they spirally wound, plate and frame,tubular, or hollow fiber elements can be employed either in series or inparallel. U.S. Pat. No. 5,238,563, herein incorporated by reference,discloses a multiple-element housing wherein the elements are grouped inparallel with a feed/retentate zone defined by a space enclosed by twotube sheets arranged at the same end of the element.

Selective membrane separation of the benzene-containing gasoline, e.g.,whether it is the full range gasoline or light cat naphtha, intopermeate and retentate is conducted under pervaporation or perstractionconditions. Preferably, the process is conducted under pervaporationconditions.

The pervaporation process typically relies on vacuum on the permeateside to evaporate or otherwise remove the permeate from the surface tothe membrane and maintain the concentration gradient driving force whichdrives the separation process. The feed is in the liquid and/or gasstate. When in the gas state the process can be described as vaporpermeation. The maximum temperature employed in pervaporation will bethat necessary to vaporize the components in the feed which one desiresto selectively permeate through the membrane, while still being belowthe temperature at which the membrane is physically damaged.Pervaporation can be performed at a temperature of from about 25° C. to200° C. and higher, the maximum temperature being that temperature atwhich the membrane is physically damaged. The feed pressure into amembrane unit is usually in the range of 1 to 20 atmospheres and theunit operated under vacuum in the range of 0.1 to 300 millimeter ofmercury. It is preferred that the pervaporation process be operated as asingle stage operation to reduce capital costs. Alternatively to avacuum, a sweep gas can be used on the permeate side to remove theproduct. In this mode the permeate side would be at atmosphericpressure.

In a perstraction process, the permeate molecules in the feed diffuseinto the membrane film, migrate through the film and reemerge on thepermeate side under the influence of a concentration gradient. A sweepflow of liquid is used on the permeate side of the membrane to maintainthe concentration gradient driving force. The perstraction process isdescribed in U.S. Pat. No. 4,962,271, herein incorporated by reference.

Whether pervaporation or perstraction, the selected membrane process caneasily be adjusted to various products of the FCC unit by changing feedflow rates or operating temperatures. This invention therefore providesadditional process control variables to help the refinery increaseyields, and still have a process that handles the benzene-containingnaphtha streams.

Very significant reductions of benzene in the naphtha are achievable bythe selective membranes according to this invention. Generally, theretentate, also referred to herein as the benzene deficient retentatewill have less than 1% benzene and as low as 100 ppm, depending on thebenzene concentration in the naphtha, membrane separation conditions,etc. The invention preferably reduces gasoline benzene such that theretentate contains less than 0.6% by weight benzene. Generally,sufficient benzene reduction is readily achievable in the retentatewhile substantially or significantly maintaining the level of gasolinenaphtha molecules in the retentate. The retentate stream is then routedto the refiner's gasoline pool where other refinery streams are combinedto form gasoline product.

The benzene enriched permeate is routed to the refinery's BTX unit(benzene, toluene, and xylene) for collection and appropriateprocessing, use and/or disposal. The permeate from this invention willcommonly have benzene (and toluene) in amounts ranging from 1 to 10% byweight benzene. Depending on the refinery, the high benzene enrichedpermeate can be routed to processes that further concentrate the streamto a substantially pure benzene for use in chemical operations, or thepermeate can be further processed to become a feedstock in reforming oralkylation processes.

Increased efficiency of the invention may be attained by fractionatingthe benzene-containing naphtha (full range or light) in a depentanizerdistillation column prior to the membrane unit for separating benzenefrom the naphtha fraction. Depending on the membrane selected, C₅olefins in the gasoline stream will usually remain in the retentate, butthese olefins can also concentrate in the permeate. Treating the naphthastream removes the C₅ olefins from the permeate, and therefore,concentrates benzene in the feedstock being directed to the membrane.Overall this should reduce the stage cut requirement. The C₅'s have beencut out, and higher benzene concentrations are removed at a fasterabsolute rate than when lower concentrations of benzene are present. Thecosts of condensing permeate without C₅'s will also be reduced. The C₅olefins recovered from the depentanizer can then be further processedfor another refinery need, or returned to the retentate stream prior toprocessing to the gasoline pool. Depentanizers columns are known in theart. The column to be used in this embodiment of the invention can berun to flash fractions at the column's top at 50° C., and bottomfractions at a temperature in the range of 150° to 200° C.

To further illustrate the present invention and the advantages thereof,the following specific examples are given. The examples are given forillustrative purposes only and are not meant to be a limitation on theclaims appended hereto. It should be understood that the invention isnot limited to the specific details set forth in the examples.

All parts and percentages in the examples, as well as the remainder ofthe specification, which refers to solid compositions or concentrations,are by weight unless otherwise specified. However, all parts andpercentages in the examples as well as the remainder of thespecification referring to gas compositions are molar or by volumeunless otherwise specified.

Further, any range of numbers recited in the specification or claims,such as that representing a particular set of properties, units ofmeasure, conditions, physical states or percentages, is intended toliterally incorporate expressly herein by reference or otherwise, anynumber falling within such range, including any subset of numbers withinany range so recited.

Example 1

A benzene selective polyurea/urethane membrane (M1) was made in atwo-step coating process. A solution was made that consists of 1.05 to 1mole ratio of toluene diisocyanate terminated polyethylene adipate and4,4′-methylenebis(2,6-diethylaniline) at 4.0% solids in dioxane. This isallowed to react overnight for 16 hours to generate a polyurea/urethanesolution. A polyacrylonitrile ultrafiltration substrate was dip coatedwith the resulting solution. The coated substrate was then transferredto a ventilated oven at 100° C. to dry and cure. In a second step, aseparate solution of toluene diisocyanate terminated polyethyleneadipate and 4,4′-methylenebis(2,6-diethylaniline) was prepared, but thistime at 2.65% solids in dioxane. This was allowed to react overnight for16 hours to generate a polyurea/urethane solution. This solution wasthen dip coated onto the earlier coated substrate, which had beenallowed to age 7 days. The newly coated substrate was again transferredto a ventilated oven at 100° C. The finished membrane was dry anddurable.

Properties for the polymeric separation layer of M1 was calculated usingthe methodology of Feimer et al. (U.S. Pat. No. 4,879,044). Calculatedwas an aromatic index=17.56, a urea index=50, the sum of (C═O+NH)/1000g=12.64, and C═O/NH value=5.34.

Example 2

A second benzene selective membrane (M2) was made in a two-step coatingprocess similar to that described in Example 1. A solution was made of1.05 to 1 mole ratio of toluene diisocyanate terminated polyethyleneadipate and 4,4′-methylenebis(2,6-diethylaniline) at 2.65% solids indioxane. This was allowed to react overnight for 16 hours to generate apolyurea/urethane solution. The same polyacrylonitrile ultrafiltrationsubstrate mentioned above was dip coated and then transferred to aventilated oven at 100° C. to dry and cure. The coated substrate wascoated a second time with the same coating above, after the firstcoating aged for 7 days. The finished membrane was dry and durable.

M2 was another polyurea/urethane membrane with more aromatic content andhigher functional group density. Calculated was an aromatic index=29.63,a urea index=50, the sum of (C═O+NH)/1000 g=12.97, and C═O/NHvalue=2.33.

Example 3

An FCC feedstock, having 25.5 API gravity, 11.94 K-Factor, 0.68%Conradson Carbon, and 0.12 wt % Nitrogen was treated in a DavisonCirculating Riser at 543° C. (1010° F.) reactor temperature, 172 kPa (25psig) reactor pressure and 704° C. (1300° F.) regenerator temperature,using a catalyst blend of 80% Ultima and 20% Olefin Ultra. Propyleneyield was 8.37 weight % on the fresh feed basis. The light C₃ and C₄compounds were separated from naphtha by a distillation column,operating with a bottom temperature of 32° C. (90° F.) and the toptemperature of −11° C. (12° F.). The recovered naphtha contained 1.1 wt% benzene.

The benzene selective membranes M1 and M2 were used to further treatthis naphtha in a pervaporation system at consisting of a feedreservoir, circulation pump, flow meters, a test cell containing amembrane sample located inside an oven, permeate collection vessels, anda vacuum pump. The permeate traps are cooled in liquid nitrogen (−195°C.). The feed stream was the FCC naphtha stripped of C₃ and C₄compounds. A membrane trial was run for each membrane M1 and M2 in thepervaporation unit at 120° C. and full vacuum to reduce the benzenelevels in this naphtha. A pressure regulator allowed the naphtha to bepressurized to 552 kPa (80 psi) and remain as liquid phase while hot.The vacuum pump generates a vacuum of less than 10 torr on the permeateside of the membrane.

Each permeate sample was collected for 1-2 hours. Collecting multiplepermeate samples generates a large stage cut. The retentate wascontinuously returned to the feed reservoir for recycle over themembrane. The retentate samples were collected at the start of eachfraction of permeate collection along with one final sample at the endof the run.

Concentrations of hydrocarbons (weight %) were determined for bothretentates and permeates using standard GC methods.

Results are shown on the graph (FIG. 2) where stage cut refers to thefraction of feed removed as permeate. M1 produced retentate with lessthan 0.6% benzene at 25% stage cut. M2, a more selective membrane,generated less than 0.6% benzene by 21% stage cut.

1. A catalytic cracking process comprising: (a) introducing ahydrocarbon feedstock into a reaction zone of a catalytic cracking unitwhich feedstock is characterized as having: an initial boiling pointfrom about 120° C. with end points up to about 850° C.; (b)catalytically cracking said feedstock in said reaction zone employing acracking catalyst, temperature, catalyst-to-oil ratio, pressure, steamdilution and space velocity such that an olefin yield from the unit isenhanced to produce a product comprising olefin, naphtha and benzene,wherein the product comprises about 6 to about 20% propylene based onthe weight of the hydrocarbon feedstock introduced in step (a) above;(c) fractionating the product into at least a light olefin-containingfraction, and a benzene-containing naphtha fraction; (d) recovering thelight olefin-containing fraction; (e) contacting the benzene-containingnaphtha fraction with a membrane having a sufficient flux andselectivity to separate a benzene-enriched permeate fraction and abenzene-deficient naphtha retentate fraction, said benzene-enrichedpermeate fraction being enriched in benzene compared to the retentate;(f) recovering the benzene-deficient naphtha retentate fraction; and (g)routing the benzene-enriched permeate fraction for further processing.2. A catalytic cracking process according to claim 1, wherein thecracking catalyst in step (b) comprises a pentasil zeolite crystalcontent in the range of 2 to about 20% by weight of the catalyst.
 3. Acatalytic cracking process according to claim 1, wherein the crackingcatalyst in step (b) comprises a catalyst additive comprising ZSM5 inthe range of 10 to about 80% by weight of the catalyst additive.
 4. Acatalytic cracking process according to claim 1, wherein thebenzene-containing naphtha fraction in step (c) comprises 0.6 to about3% by weight benzene.
 5. A catalytic cracking process according to claim1, wherein the benzene-containing naphtha fraction in step (c) is lightcat naphtha having a boiling point in the range of 50° to 105° C., andcomprises 1.2 to about 6% by weight benzene.
 6. A catalytic crackingprocess according to claim 2, wherein the benzene-containing naphthafraction in step (c) is light cat naphtha having a boiling point in therange of 50° to 105° C., and comprises 1.2 to about 6% by weightbenzene.
 7. A catalytic cracking process according to claim 1, whereinthe process is a deep catalytic cracking process, and the conditions inthe reaction zone of step (a) are as follows: Temperature, ° C. 505-575Cat./Oil  9 to 15 Pressure, atmospheres 0.7 to 1.5 Steam Dilution, wt %of feed 10 to 30 WHSV, hr⁻¹ 0.2-20  Pentasil crystal content, 10-75 % byweight catalyst


8. A catalytic cracking process according to claim 1, wherein themembrane in step (e) comprises a member selected from the groupconsisting of polyimide, polyurea-urethane, polysiloxane andcombinations thereof.
 9. A catalytic cracking process according to claim2, wherein the membrane in step (e) comprises a member selected from thegroup consisting of polyimide, polyurea-urethane, polysiloxane andcombinations thereof.
 10. A catalytic cracking process according toclaim 5, wherein the membrane in step (e) comprises a member selectedfrom the group consisting of polyimide, polyurea-urethane, polysiloxaneand combinations thereof.
 11. A catalytic cracking process according toclaim 1, where in the benzene-containing naphtha fraction is contactedwith the membrane under pervaporation conditions.
 12. A catalyticcracking process according to claim 1, wherein the benzene-deficientnaphtha retentate comprises less than 1% by weight to about 100 ppmbenzene.
 13. A catalytic cracking process according to claim 1, whereinthe benzene-deficient naphtha retentate comprises less than 0.6% byweight to about 100 ppm benzene.
 14. A catalytic cracking processaccording to claim 1, wherein the benzene-enriched permeate comprises 1to about 10% by weight benzene.
 15. A catalytic cracking processaccording to claim 1 further comprising fractionating thebenzene-containing naphtha fraction from step (c) to remove C₅ fractionsprior to contacting the benzene-containing naphtha fraction with amembrane in accordance with step (e).
 16. A method of producing lightolefins and low benzene-containing naphtha in a catalytic cracking unitfrom a hydrocarbon feedstock, the method comprising: (a) selectingcatalyst, temperature, catalyst-to-oil ratio, pressure, steam dilutionand/or space velocity to enhance olefin yield in the catalytic crackingunit; (b) contacting the feedstock with cracking catalyst in the unit,thereby producing a product comprising olefin, naphtha and benzene; (c)fractionating the product into at least a light olefin-containingfraction, and a benzene-containing naphtha fraction; (d) recovering thelight olefin-containing fraction; (e) contacting the benzene-containingnaphtha fraction with a membrane having a sufficient flux andselectivity to separate a benzene-enriched permeate fraction and abenzene deficient naphtha retentate fraction, said benzene-enrichedpermeate fraction being enriched in benzene compared to the retentate;(f) recovering the benzene deficient naphtha retentate fraction; and (g)routing the benzene-enriched permeate fraction for further processing.17. A method according to claim 16, wherein the benzene-containingnaphtha fraction in step (c) has a boiling point in the range of 50° C.to about 220° C., and the fraction comprises 0.6 to about 3% by weightbenzene.
 18. A method according to claim 16, wherein thebenzene-containing naphtha fraction in step (c) has a boiling point inthe range of 50° C. to about 105° C., and the fraction comprises 1.2 toabout 6% by weight benzene.
 19. A method according to claim 16, whereinthe product from step (b) comprises 6 to about 20% by weight propylenebased on the weight of the feedstock in step (a).
 20. A method accordingto claim 16, wherein the membrane in step (e) comprises a memberselected from the group consisting of polyimide, polyurea-urethane,polysiloxane and combinations thereof.
 21. A method according to claim16 wherein the benzene-containing naphtha fraction is contacted with themembrane under pervaporation conditions.
 22. A method according to claim16, wherein the benzene-deficient naphtha retentate comprises less than1% by weight to about 100 ppm benzene.
 23. A method according to claim16, wherein the benzene-deficient naphtha retentate comprises less than0.6% by weight to about 100 ppm benzene.
 24. A method according to claim18, wherein the benzene-deficient naphtha retentate comprises less than1% by weight to about 100 ppm benzene.
 25. A method according to claim19, wherein the benzene-deficient naphtha retentate comprises less than1% by weight to about 100 ppm benzene.
 26. A method according to claim16, wherein the feedstock is contacted with a catalyst comprising ZSM5.27. A method according to claim 16, wherein the feedstock in step (b) iscontacted with a catalyst comprising 2 to about 20% by weight pentasilcrystal.
 28. A method according to claim 19, wherein the feedstock instep (a) is contacted with a catalyst comprising 2 to about 20% byweight pentasil crystal.
 29. A method according to claim 27, wherein thepentasil is ZSM-5.
 30. A method according to claim 28, wherein thepentasil is ZSM-5.
 31. A method according to claim 16, wherein thecracking catalyst in step (b) comprises a catalyst additive comprisingZSM5 in the range of 10 to about 80% by weight of the catalyst additive.32. A method according to claim 19, wherein the cracking catalyst instep (b) comprises a catalyst additive comprising ZSM5 in the range of10 to about 80% by weight of the catalyst additive.